An imaging device, such as a time-of-flight (TOF) three-dimensional (3D) camera, may include a light source for illuminating a scene containing one or more objects for imaging. The imaging device may collect light reflected from the objects on a photosensitive surface. An amount of light that the photosensitive surface receives per second per unit area (i.e., irradiance) depends in part upon the location of the objects in the scene, or in the example of a TOF 3D camera, the location of the objects in the camera's field of view. For an object having an angular displacement relative to an optical axis of the camera, the irradiance at the photosensitive surface will generally decrease as the angular displacement increases.
For TOF 3D cameras and other applications of imaging devices, it may be advantageous for objects in a scene to have a substantially equal irradiance on the photosensitive surface independent of the objects' angular displacement relative to the camera's optical axis. To compensate for a decrease in irradiance with angular displacement of an object in a scene, a camera illumination system may be configured to increase illumination of regions of the scene as a function of the region's increasing angular displacement from the optical axis. As a result, features having greater angular displacement are illuminated with more intense light.
An example illumination system that increases illumination of a region as a function of the angular displacement of the region may include a collimator that collimates light from a light source. A diffractive diffuser receives the collimated light and distributes the light across the camera field of view to compensate for the decrease in irradiance with angular displacement. However, diffractive diffusers are relatively inefficient and may deliver less than 75% of the light they receive from the light source to the camera field of view. Additionally, illumination systems are relatively expensive, and the costs and engineering difficulty associated with dissipating heat these systems generate increase with the amount of light they produce. Accordingly, the intensity of illumination produced by illumination systems is usually limited by cost considerations and heat dissipation requirements. Additionally, for applications that benefit from threshold irradiance values, such as a TOF 3D camera, the relative inefficiency of conventional camera illumination systems combined with the concomitant increase in costs and engineering difficulty associated with higher light output, may limit the operating precision capabilities of such applications.